According to the game's producer at Blizzard, Chris Sigaty, the decision to break up the game was not due to a request from the newly merged Activision Blizzard executive team. In fact the decision was made last year before they knew of the coming merger. Sigaty states, "We were running up against basically having a huge story with big scope." So instead of cutting elements our of Starcraft II they decided to try a new way of presenting the type of story they wanted. "And because it wouldn't affect multiplayer at all, and that each product would have upgrades to multiplayer - expanding and improving upon multiplayer - we thought it was a positive move."
The first game, Terrans: Wings of Liberty will be followed by Zerg: Heart of the Storm and finally Protoss: Legacy of the Void. Sigaty states that all three are considered to be full games at least on the single player level but declined to say how much each game would cost. Also it could be a year or even more between releases of each title althought he does state, "We want to hit the shortest amount of time possible." Of course there's still no release date for the first title.
Gallery: StarCraft 2 Screenshots